1. Field of the Invention
The present invention relates to an ink jet recording apparatus which discharges ink as ink droplets.
2. Discussion of the Background
Conventionally, an ink jet recording apparatus such as an ink jet printer performs image formation by discharging ink in an ink jet head as ink droplets from nozzles to a recording medium.
In this ink jet recording apparatus, when the nozzles are clogged with foreign materials or bubbles occur in the ink jet head, ink discharge becomes unstable, or even ink discharge failure occurs, which further causes a problem of poor printing.
To solve the problem, proposed is a method of providing a filter for filtration of ink in an ink supply channel communicating an ink tank with the ink jet head (See United States Patent Application Publication No. US 2002/0012034 A1 (Japanese Patent No. 3168122)). The filter removes foreign materials or the like from the ink. Further, as the filter is slanted to a horizontal surface so as to trap bubbles flowed with the ink from the ink tank, the filter prevents the bubbles from flowing into the ink jet head.
Further, proposed is a method of providing a filter, which removes only foreign materials and passes bubbles, and sucking a nozzle portion of ink jet head, thereby removing the bubbles from the ink jet head (See U.S. Pat. No. 5,828,395 (Japanese Published Unexamined Patent Application No. Hei 9-94978)).
On the other hand, as a characteristic of the filter used in these methods, the absolute filter diameter must be small to trap foreign material of predetermined size. However, in a case where pigment ink is used in the ink jet recording apparatus, the filter may remove the pigment component of the ink, and the filter may be clogged with the pigment component. Generally, an average particle diameter of pigment is about 0.05 to 0.50 μm, however, a pigment particle having a diameter greater than 1 μm exists. The filter used in the ink jet recording apparatus is generally formed by weaving a fine metal line or by metal sintering. Accordingly, the trapping rate of the filter differs by particle diameter, and it varies as shown in a graph of FIG. 8.
In the case where the pigment ink is used in the ink jet recording apparatus, to avoid clogging in the filter, the trapping rate of the filter for the maximum pigment particle diameter must be 0%. In the above-described filter, if the maximum particle diameter is 2 μm, the trapping rate for 2 μm or fewer diameters must be 0%, as shown in FIG. 8, and the trapping rate increases from a particle diameter greater than 2 μm. When a foreign material of 5 μm in size is included in the ink, the material is trapped by the filter with a constant probability. The trapped material is fitted into a clearance of the filter, thus the mesh of the filter is clogged. In the filter of this state, pigment with a particle diameter of 2 μm or less may be trapped. In such case, the trapping rate of the pigment having a particle diameter of 2 μm or less is not 0%.
Further, there is a probability that when the pigment is trapped by the filter, the pigment ink does not satisfy the specification as ink. Further, the trapped pigment accelerates clogging in the filter. On the other hand, in a case where a foreign material of about 100% trapping rate, e.g., a material of about 10 μm in size is included in the ink, the material may break through the filter and have a bad influence within the ink jet head.
In the method disclosed in the U.S. patent application Publication No. US 2002/0012034 A1 (Japanese Patent No. 3168122), as the bubbles flowing with the ink from the ink tank are trapped by the filter, there is no influence on the performance of the ink jet head. However, bubbles occur around the nozzles of the ink jet head, from bubble drawing from the nozzles due to temperature/pressure change, or from a gas dissolved in the ink. The bubbles occurred inside the ink jet head cannot be removed by the above method disclosed in the U.S. patent application Publication No. US 2002/0012034 A1 (Japanese Patent No. 3168122).
Further, in a structure where the filter is horizontally provided, bubbles moved from the ink jet head stay on the filter surface. Even if the amount of bubbles is small, the bubbles spread there form a thin bubble layer. The bubble layer, as a cushion, disturbs ink supply, thus seriously influences ink discharge.
Note that even in the structure in which the filter is slanted, as disclosed in the U.S. patent application Publication No. US 2002/0012034 A1 (Japanese Patent No. 3168122), as bubbles stay on the filter surface and the amount of bubbles gradually increases, an effective filtering area of the filter is narrowed, and the speed of ink filtering is reduced. Further, pressure control necessary for ink supply becomes difficult.
Further, in the method disclosed in the U.S. Pat. No. 5,828,395 (Japanese Published Unexamined Patent Application No. Hei 9-94978), there is a high probability that bubbles are lead in the nozzle portion of the ink jet head. As the bubble absorbs a pressure impact, the mixture of bubble in the nozzle portion seriously degrades the performance of the ink jet recording apparatus.